The steel and aluminum tariffs would also hit Britain, Germany, South Korea, Turkey and Japan, countries with which the United States has close national security ties.
The EU has threatened retaliatory tariffs on US manufactured goods, including motorcycles and blue jeans, and the worry is that the tit-for-tat will harm American businesses that export, while raising costs for businesses that rely on a global supply chain.
Navarro said on Fox that the national security and economic security provision Trump cited in announcing the 25 per cent tariffs on steel and 10 per cent on aluminum is "country-agnostic," meaning that it applies across the board.
But speaking on CNN, Navarro suggested that there could be exemptions in "particular cases where we need to have exemptions so that business can move forward."
However, in the same interview, he also pushed back on the Pentagon's argument that the new trade policy could have an adverse effect on national security, arguing against giving allies a pass on the tariffs because "as soon as you exempt one country, then you have to exempt another country."
Allies are concerned about the tariffs because they supply more steel directly to the US than China - which dominates a major part of the global steel trade and Trump's rhetoric.
"If you exempt Canada, then you have to put big tariffs on everybody else," Navarro said on CNN.
Trump is facing criticism from members of his own party over his trade plan, including from some of the top economists who advised his campaign. Navarro brushed off the criticism, arguing that Trump was alone in the GOP on trade during the 2016 presidential campaign - and won anyway.
"All 16 of those candidates didn't agree with his policies, either," Navarro said on CNN. "They're dead wrong on the economics, there's no downstream effect here. There's only a president ... saving and defending our aluminum and steel industries."
Commerce Secretary Wilbur Ross, appearing on ABC's This Week, said he expects the formal rollout of the tariffs this week.
"I have not heard him describe particular exemptions," Ross said.
As for speculation that White House economic adviser Gary Cohn might quit over the new policy, with which he apparently disagrees, Ross said Trump likes to hear opposing views before he makes decisions.
"Gary Cohn, as far as I know, is certainly not going to walk out," Ross said on ABC.
In an interview on NBC, Ross said he does not think that Trump will reverse himself under pressure, but he left room for the possibility that the unpredictable president could change his mind.
"Whatever his final decision is, is what will happen," Ross said. "What he has said, he has said. If he says something different, he will do something different."
Navarro cast the decision as a national security imperative to protect vital manufacturing industries at home and said Trump is acting to fulfill a campaign promise to US workers.
Economists warn that any benefit in terms of jobs could be far outweighed by increased steel costs for US automobiles, wind turbines, shale oil and gas drilling rigs, and more.
Ohio Governor John Kasich, a Republican, also appearing on CNN, excoriated the President over the planned tariffs.
"You just don't do things like that off the cuff," Kasich argued. "Trade wars that divide us from our allies make no sense."
